Virtual1 (en-GB)

Bits in Space


James Hickman - CTO at Virtual1

As Tim Peake is settled into life at the International Space Station, I got to wondering about all the home comforts an astronaut must leave behind when they are off to live in a tin can hurtling around the Earth for months at a time.Trips to see friends would be a little complicated, they only recently got a coffee machine up there and privacy pretty much is non-existent. But surely they have the Internet? They most certainly do. But they get jealous of people living in deepest darkest Norfolk with their 1Mbps ADSL lines. But it isn’t about the bandwidth because they have a 10Mbps connection.

It seems that there haven’t been enough requirements to hurriedly solve a debate about some trivial fact and thus Google wasn’t top of NASA’s priority list. I bet the ban on alcohol is the main reason behind that, but I digress.

Internet on the ISS is based on satellite communications, which needs a bit of explanation. You see the ISS is underneath the satellites, just like we are. Any click on a website goes up about 20,000 miles to find a satellite and then down to Earth. For the technically minded, this takes around 400ms or just under half a second. And there is the problem.

Latency is one of those things that is often mentioned but not always understood. Latency is the time it takes for a packet of data to get from the sender to the receiver. This is one-way and is affected by how long it takes every device in the path to pass it on to the next one in the chain, but mainly latency is about distance and what you are travelling through. Air is slower than copper and copper is slower than fibre. When you go from sender to receiver and back to the sender, this is your “Round Trip Time” or RTT and will usually be at least twice the one-way latency if the path is the same. You can easily find your RTT by using a simple PING command.

When you are doing something non-interactive like email or sending a file, the latency makes things take longer but is bearable, for interactive services like voice or video it can be a real pain. For IP voice, quality degradation is audible above 250ms with 150ms the maximum limit for any kind of decent quality. The upper limit for it to work at all is 500ms, but you’ll be talking over each other unless you learn to say “over” when you finish talking. Luckily astronauts know how to do that, it would just seem odd in a conference call with your customers.

Some tricks that can be used to improve the TCP/IP layer by spoofing the regular“did you get that?” control messages, compressing some of the data and making the windows between checking a bit longer. These don’t generally work that well for voice though, so your best bet is always to use the best quality network hardware you can, apply QoS but most of all, choose a good underlying network technology. When it comes to terrestrial networks, fibre Ethernet should be your first choice.

And NASA? Well they can’t quite get a fibre optic cable strung between the ISS and the ground just yet so they are going to start using a laser. Just like we use in every fibre-based circuit down here on Earth. I bet those astronauts will start subscribing to Netflix as soon as they get it working.